An example of a conventionally known actuator with a lock mechanism is illustrated in FIG. 3. The actuator comprises a cylinder 3, a piston 4 slidably mounted in the cylinder 3, and a lock mechanism 10. The cylinder 3 has inlet and outlet ports 1 and 2 for supplying and exhausting working fluid and a front wall disposed at the front end thereof. The piston 4 includes a piston head 5 and piston rod 7 connected to the piston head 5. The lock mechanism 10 temporarily locks the piston 4 to the cylinder 3 by engaging a key 9 with a key groove 8 formed at an outer periphery of base portion of the piston rod 7, when the piston head 5 reaches the extended stroke end A, i.e., a position away from an inner surface 6 of the front wall by a predetermined distance L.
When such an actuator with a lock mechanism is used as, for example, an actuator for taking in and out wheels of an aircraft, a longitudinal hole 11 extending from the piston head 5 toward the front end is usually formed in the piston 4 so as to make the piston hollow as illustrated in FIG. 4 in order to lessen the weight of the actuator. In order to minimize the weight, it is preferable that the inner diameter of the longitudinal hole 11 is as large as possible. However, in order to maintain the strength of the piston, it is necessary that the thickness of the piston rod 7 is almost the same over the entire region of the piston rod 7. In order to satisfy both the above-described requirements regarding minimization of the weight and the sufficient strength, it is necessary that the longitudinal hole 11 includes a shoulder therein, i.e., the diameter of the longitudinal hole 11 is set small near the key groove 8 and is set large at the remaining region. Accordingly, there is a problem that the boring operation of the longitudinal hole 11 is troublesome, and that correspondingly, the manufactured actuator becomes expensive.